In an image forming apparatus using an electrophotographic method, a single-component developing method using toner alone as developer and a two-component developing method using both toner and carrier as developer have been known as a development method for developing an electrostatic latent image formed on an image carrier.
In the single-component developing method, generally, toner is made to pass through a regulating section formed by a toner carrier and a regulating plate pressed against the toner carrier, whereby the toner is charged and a desired thin toner layer is obtained. This method has advantages of simplification, downsizing and cost reduction of the apparatus.
In the meantime, toner deterioration tends to be accelerated by the heavy stress at the regulating section, and toner charge-acceptance ability tends to be reduced. Further, the regulating member as a charge providing member for providing charge to the toner and the surface of the toner carrier are contaminated with the toner or external additive agent, whereby the charge-providing ability for providing charge to the toner is also reduced. This will reduce the amount of toner charge and will cause fogging and related problems, with the result that the service life of the development device is reduced.
Comparison reveals that, the two-component developing method is advantageous to realize a longer service life since the toner is mixed with a carrier to be charged by triboelectric charging, thereby causing less stress, and since the carrier is not easily contaminated with toner or external additives because of a greater area of its surface.
However, in the two-component developing method, when an electrostatic latent image on the image carrier is to be developed, the image carrier surface is brushed by a magnetic brush formed of the developer. This may create a problem that a mark of the magnetic brush remains on a developed image. Further, the carrier tends to be attached to the image carrier, whereby an image defect occurs.
The so-called hybrid development method was disclosed (e.g., Japanese Patent Application Publication No. H05-150636) as a development method that provides image quality as high as that of the single-component developing method, and solves the problem of image defect, and this method is characterized by a long service life achieved by the two-component developing method using a two-component developer. In this hybrid development method, a two-component developer is carried on the developer carrier, and only the toner is supplied from the two-component developer to the toner carrier.
However, the hybrid development method described in the Japanese Patent Application Publication No. H05-150636 includes an issue of development hysteresis (ghost) as described below.
The issue of development hysteresis (ghost) is an issue, which the hybrid development method generally has, and in which post-development residual toner which is not used for development is deposited on an image as a development hysteresis (ghost), in the next development step.
At the facing portion (toner supplying and recovering area) of the toner carrier and the developer carrier provided to supply toner to the toner carrier, a bias is applied to supply the toner, and the recovering of the post-development residual toner is also carried out at the same facing portion to the developer carrier.
As above-mentioned, a bias voltage applied in the supplying direction for supplying the toner in the supplying-recovering zone; such a constitution becomes as the factor hindering the toner recovery so that the toner recovering ability becomes insufficient. Consequently, a portion having larger amount of the post-development residual toner and a portion having smaller amount of the post-development residual toner appears as a contrast of density at the next developing process.
Techniques for reducing such a development hysteresis (ghost) have been developed particularly in the constitution of the facing zone (nip portion) of the developer carrier for supplying (and recovering) the toner and the toner carrier (refer to, for example, Unexamined Japanese Patent Application Publication 2003-316155).
In the development device described in Unexamined Japanese Patent Application Publication 2003-316155, the following setting is disclosed as a constitution of a nipping portion (toner supplying-recovering area): the rotating direction of the developing roller (toner carrier) and that of the magnetic brush roller (developer carrier) are opposite to each other; and the position of magnetic pole of the magnetic brush roller facing the developing roller is shifted at 0 to 15° toward the upstream of the rotating direction of the magnetic brush roller from the closest position.
As above-mentioned, the constitution capable of maintaining both of the toner supplying ability and toner recovering ability at the nip portion at an appropriate level is required for reducing the occurrence of development hysteresis (ghost).
In the development device described in Unexamined Japanese Patent Application Publication 2003-316155, the nip portion is separated into the toner supplying portion and the toner recovering portion by setting the rotating direction of the toner carrier and the toner carrier to be opposite.
Moreover, the toner supplying ability on the entrance side of the toner supplying nip portion is improved by positioning the brushing peak of the magnetic brush on the upstream side of the rotating direction of the developer carrier from the closest position.
Supplying toner from the developer generates counter charge opposite to the toner charge in the developer. The counter charge hinders the supply of toner. Since the counter charge generation is unavoidable, therefore, the constitution described in Unexamined Japanese Patent Application Publication 2003-316155 is configured so that most of the toner is supply at the initial period of generation of counter charge in the toner supplying portion, thereby keeping the toner supplying ability.
On the other hand, the counter charge accelerates the recovery of toner. However, according only to the configuration described in Unexamined Japanese Patent Application Publication 2003-316155, the generated counter charge is not effectively utilized in the toner recovering portion, there failing to achieve sufficient toner recovering ability.
Therefore, the occurrence of the development hysteresis (ghost) is sufficiently reduced with that configuration.